1. Field of the Invention
The invention relates to a bicycle wheel rim, more particularly to a rim of a bicycle wheel that can ensure proper frictional contact with a brake mechanism on a bicycle frame even after adjustment of the wheel axle relative to the bicycle frame has been performed to ensure tight engagement between a drive chain and a chain wheel for driving rotation of the bicycle wheel.
2. Description of the Related Art
Referring to FIG. 1, after a bicycle 1 has been in use for a period of time, the drive chain 11 thereof might become loose and slightly lengthen. At this time, the drive chain 11 might be easily and undesirably released from the chain wheel 120 to result in inconvenience when the bicycle 1 is in use. As such, an adjustment device 13 has been proposed to permit adjustment of the position of the wheel axle 121 of the rear bicycle wheel 12 on the bicycle frame. The adjustment device 13 is mounted on the seat stay 141 and the chain stay 142 of the bicycle frame, and includes a plate member formed with an elongated slot 131 for retaining the wheel axle 121 therein. When the drive chain 11 becomes loose, the wheel axle 121 can be moved along the slot 131 to permit the drive chain 11 to engage fittingly the chain wheel 120.
However, since the brake mechanism 15 of the bicycle 1 is mounted on the seat stay 141, and since the wheel rim 122 of the rear bicycle wheel 12 moves together with the wheel axle 121 during the adjustment operation, unless a corresponding adjustment is made for the brake mechanism 15, the brake pads 151 of the brake mechanism 15 cannot be kept in proper frictional contact with brake pad contacting surfaces formed on the tire retaining walls of the wheel rim 122 after the wheel axle 121 has been moved to an adjusted position, as shown in phantom lines in FIGS. 2 and 3.
In co-pending U.S. patent application Ser. No. 09/321,467, there is disclosed a bicycle wheel rim that is adapted for use with a brake pad having a width in a radial direction of the bicycle wheel rim, and that is adapted for mounting a plurality of spokes thereon. The bicycle wheel rim includes spaced left and right annular tire retaining walls that are adapted for retaining a bicycle tire therebetween. Each of the tire retaining walls has an inner edge proximate to a central point of the wheel rim, an outer edge distal to the central point of the wheel rim, and an intermediate portion between the inner and outer edges. Each of the tire retaining walls further has an outer brake pad contacting surface that has a width in the radial direction of the wheel rim. The width of the brake pad contacting surface of each of the tire retaining walls is at least twice the width of the brake pad. The bicycle wheel rim further includes an annular spoke mounting wall that interconnects the inner edges of the tire retaining walls and that has a central spoke fastening portion which extends along a length of the spoke mounting wall and which is formed with a set of spoke fastening holes adapted for mounting the spokes thereat.
Because the width of the brake pad contacting surface of each of the tire retaining walls is at least twice the width of the brake pad, after the wheel rim moves together with the wheel axle to an adjusted position where the drive chain can engage fittingly the chain wheel, the brake pad can still be kept in proper frictional contact with the brake pad contacting surface of the adjacent tire retaining wall. However, recent advancements in the materials used for fabricating the brake pads and the wheel rim, and in the surface treatment of the brake pad contacting surfaces of the tire retaining walls, have resulted in higher friction coefficients, thereby resulting in a larger braking force per unit area. Thus, the length of the brake pad can be reduced. Under such a condition, the requirement of the width of the brake pad contacting surface of each of the tire retaining walls to be at least twice the width of the brake pad is overestimated.